Toronto to Montreal and/or Ottawa of course. And yes, part of the motivation is that air travel can be a pain. Even the slow trains we have now compete somewhat effectively against air for middle distance (300-500 km) travel, because they go downtown to downtown and are well served by hotels, restaurants and conference facilities often within walking distance or a very short cab ride. In fact even air works best if there is a local train at each end. Also a more comfortable seat, faster loading and unloading, easy to get up and stretch your legs, and (so far, thank God) much easier security procedures. It only takes a bit longer and it is more useable time, not as fractured as air travel time with its landing and takeoff precautions around using electronics, headphones etc.

Also airports are hugely expensive infrastructure too, and have a huge impact on surrounding properties, with a lot of resistance to expansion, and there are or will be various limiting factors in how many flights can take off and land for a particular route; so when the flight corridors or runways or boarding gates that connect Dallas and Houston get saturated, and the highways are in gridlock too, one way to increase capacity, and flexibility is by adding a rail option.